A high-quality containerized culinary herb is one that is proportional to its container with a compact growth habit and stems or leaves that are not excessively elongated. However, chemical plant growth regulators (PGRs) used to modify plant growth, increase branching, or reduce stem elongation are not labeled for use on culinary herbs. A novel option for reducing stem elongation could be exposure to night-interruption (NI) ultraviolet (UV) radiation. Therefore, our objectives were to 1) quantify the influence of UV-A radiation on the growth, stem elongation, and relative chlorophyll concentration of containerized spearmint (Mentha spicata) and Greek oregano (Origanum vulgare var. hirtum) and 2) determine the most effective duration and sequence of exposure. Rooted cuttings were transplanted into 15-cm containers and placed in a greenhouse at 23 °C and under a 16-h long-day (LD) provided by LED supplemental lighting (SL) or under a 9-h short-day (SD) and 6-h of NI lighting providing 385 nm of UV-A radiation at an intensity of 20 µmol∙m–2∙s–1 for 4 weeks. The 10 treatments included: LD entire time, LD 2 week-UV 2 weeks, LD 1.5 weeks-UV 2.5 weeks, LD 2 weeks-UV 2 weeks, LD 2 week-UV 1 weeks-LD 1 week, UV entire time, UV 2 weeks-LD 2 weeks, UV 2.5 weeks-LD 1.5 weeks, UV 2 weeks-LD 2 weeks, or UV 2 week-LD 1 week-UV 1 week. Exposure to SD UV-A NI for 4 weeks, resulted in a ≈ 25 and 29% reduction in height of Greek oregano and spearmint, respectively. Furthermore, when LDs were the last treatment spearmint plants received, they had a higher chlorophyll content than those exposed to UV-A. Our results indicate that UV-A NI lighting for spearmint and Greek oregano may be an option for height control in controlled environments.
